Blood pressure monitoring is a fundamental diagnostic tool for a wide array of cardiovascular and other pathologic conditions. Systolic and diastolic blood pressure may be measured indirectly and non-invasively using a standard cuff method. However, direct monitoring of blood pressure is sometimes desired for diagnosing or tracking a disease state using relatively invasive measurement methods that use an indwelling blood pressure catheter. An indwelling catheter, however, may only remain implanted for relatively acute periods of time to prevent infection or other associated complications. Chronic monitoring of blood pressure could be extremely valuable to a physician in numerous patient monitoring applications, such as monitoring heart failure, hypertension, diabetes, or other infrequent, acute conditions such as unexplained syncope, unexplained seizures, etc.
Chronic blood pressure monitoring can be performed by placing an implantable pressure sensor directly in the cardiovascular system, such as the right ventricle. An implantable absolute pressure monitoring system is disclosed in U.S. Pat. No. 5,535,752, issued to Halperin, et al. A chronically implantable device for monitoring the intracardiac electrogram (EGM) and right ventricular blood pressure has been implanted in chronic heart failure patients and used to continuously monitor their hemodynamic condition. See Magalski A, et al., J. Card. Fail., 2002;8(2):71-3. Apparatus for monitoring for syncope including electrodes for recording electrical activity and/or blood pressure data of the patient's heart is disclosed in U.S. Pat. No. 6,351,670 issued to Kroll. An ambulatory cardiac diagnostic unit is disclosed in PCT Publication No. WO 90/04942, issued to Baker et al., which monitors heart action using intracardiac electrogram and pressure sensed using an intracardiac lead. These systems advantageously provide chronic blood pressure monitoring but involve placement of a lead within the patient's heart.
Placement of a pressure sensor directly in the heart or cardiovascular system is a relatively invasive procedure with associated risks and complications. Therefore, it is desirable to provide a device and method for chronically monitoring blood pressure that is a simple to implant, relatively non-invasive device. Indirect methods of measuring blood pressure allow a sensor to be placed outside of the cardiovascular system and the measured signal may be correlated to blood pressure.
FIG. 1 is a graphical representation of simultaneous ECG events, blood pressure changes and heart sounds that occur in the left ventricle during a cardiac cycle. Ventricular systole begins when an action potential conducts through the atrioventricular node (AV node) and quickly depolarizes the ventricular myocardium. This event is distinguished by the QRS complex on the ECG. As the ventricles contract, pressure in the ventricles begins to rise, causing abrupt closure of the mitral and tricuspid valves between the ventricles and atria as ventricular pressure exceeds atrial pressure. As illustrated in FIG. 1, this valve closure generates the first heart sound, S1, at the start of ventricular systole. S1 generally has a duration of about 150 ms and a frequency on the order of 25 to 45 Hz. In FIG. 1, left ventricular pressure (LVP) is seen to rise dramatically following the QRS complex portion of the ECG and closure of the mitral valve.
The ventricular pressure continues to build until the aortic and pulmonary valves open, ejecting blood into the aorta and pulmonary artery. In FIG. 1, aortic pressure is seen to rise with left ventricular pressure after opening of the aortic valve. Ventricular contraction continues to cause blood pressure to rise in the ventricles and the aorta and pulmonary artery. As the contraction diminishes, blood pressure decreases until the aortic and pulmonary valves close. Aortic pressure typically reaches a maximum of 120 mmHg during ventricular systole and a minimum of 80 mmHg during ventricular diastole. The second heart sound, S2, corresponds to closure of the aortic and pulmonary valves, near the end of ventricular systole and start of ventricular diastole. S2 generally has a duration of about 120 ms and a frequency on the order of 50 Hz and will be related to the diastolic pressure in the aorta and the pulmonary artery.
The third heart sound, S3, is associated with early, passive diastolic filling of the ventricles, and the fourth heart sound, S4, is associated with late, active filling of the ventricles due to atrial contraction. The third sound is generally difficult to hear in a normal patient using a stethoscope, and the fourth sound is generally not heard in a normal patient. Presence of the third and fourth heart sounds during an examination using a stethoscope may indicate a pathological condition. Physicians are particularly familiar with evaluating heart sounds as part of a basic physical examination, and a stethoscope is a standard component in a physician's diagnostic tool box.
An extravascular hemodynamic monitor that includes vascular plethysmography, heart and lung sound, thoracic impedance and ECG sensors is generally disclosed in U.S. Pat. No. 6,409,675, issued to Turcott, incorporated herein by reference in its entirety. Phonocardiogram data is stored for later retrieval allowing the physician to review the morphology of the phonocardiogram for signs of acute heart failure exacerbation.
Mathematical models for calculating systolic and diastolic blood pressure based on the frequency and/or amplitude components of the first and second heart sounds have been proposed. See, for example, A. Bartels et al., “Non-invasive determination of systolic blood pressure by heart sound pattern analysis,” Clin. Phys. Physiol. Meas., 1992; 13:249-56; D. Chen et al., “Estimation of pulmonary artery pressure by spectral analysis of the second heart sound,” Am. J. Cardiology, 1996; 78:785-9; and J. Xu et al., “A new simple and accurate method for non-invasive estimation of pulmonary arterial pressure,” Heart, 2002; 88:76-80.
Therefore, chronic monitoring of blood pressure based on heart sounds could provide physicians with a valuable diagnostic tool for detecting and monitoring pathologic conditions, which manifest in abnormal blood pressure responses or changes. For example, syncope, the temporary loss of consciousness due to cerebral ischemia, can be particularly problematic to diagnose because a syncopal event can be sudden, without warning, and may occur very infrequently, rarely under the supervision of an examining physician. Underlying causes of syncope can be associated with cardiac causes, metabolic causes, neurologic causes, and other miscellaneous causes such as coughing, severe pain or vertigo.
Cardiac causes include a group of autonomic disturbances resulting in othostatic intolerance. The heart rate is normally regulated by a balance between the sympathetic and parasympathetic (vagal) components of the autonomic nervous system. Increased sympathetic activity increases the heart rate and has vasoconstrictive effects that increase blood pressure. Increased parasympathetic activity decreases the heart rate and has vasodilative effects that decrease blood pressure. A positional change to an upright position is normally responded to by an increase in sympathetic output and a decrease in parasympathetic output to compensate for the gravitational effect that displaces blood to the lower extremities. Any failure in this normal reflex can impair cerebral perfusion resulting in syncope.
Because of the numerous etiologies of syncopal events, syncope can go unexplained in many patients, preventing proper treatment. Underlying causes could be life-threatening cardiac conditions. Even when the cause is benign, serious injury can occur due to falling during a syncopal event or if a syncopal event occurs during driving. Recurrent, unpredicatable loss of consciousness can have a profound effect on a patient psychologically and on his or her quality of life.
Evaluation of a patient experiencing syncope can be extensive and costly and include ambulatory ECG (Holter) monitoring, echocardiography, electrophysiologic testing, exercise-tolerance testing, electroencephalography, computed tomography, coronary angiography, glucose-tolerance testing and tilt table testing. Tilt table testing is performed in an attempt to induce a syncopal event by subjecting a patient to a positional change and monitoring the patient's blood pressure and heart rate to determine abnormalities in the autonomic reflex. Conclusive diagnosis may remain elusive even after extensive clinical examination because the precipitating factors leading to a syncopal event may not be present during an examination.
ECG monitoring using an ambulatory device (Holter), does not always reveal a cardiac cause of syncope because a patient may wear a Holter monitor only for a period of one or two weeks, during which no syncopal events occur, and patients may be non-compliant in wearing an external device. Chronic ECG monitoring using a minimally invasive implantable device, such as the Reveal® insertable loop recorder available from Medtronic, Inc., Minneapolis Minn., can be performed for longer periods of time, for more than one year, with little inconvenience to the patient. Such chronic ECG monitoring can reveal cardiac arrhythmias at the time of infrequent, spontaneous syncopal events, allowing a physician to make a diagnosis.
Cardiac arrhythmias, however, may not be the sole or initiating cause of syncope. Hypotensive episodes may precede an arrhythmic event, or be present during sustained normal sinus rhythm, and still lead to syncope or near syncope. Therefore a combination of chronic blood pressure monitoring and ECG monitoring would provide improved diagnostic information for a physician. Such chronic monitoring would be valuable not only in monitoring a patient having unexplained, recurrent syncope but also in patients suspected of having transient arrhythmias, cardiomyopathy or other cardiac conditions, blood sugar fluctuations, respiratory conditions, or other conditions that may manifest in hypotensive or hypertensive episodes. Gathered ECG and blood pressure data can be useful for diagnostic purposes as well as evaluating the effectiveness of a prescribed therapy.